Eddy currents provide a measurable indicator of flaws in the surface and sub-surface of conductive materials. They are generally confined to the surface and near surface regions of the material. The eddy currents are affected by changes in the resistivity of the conductive material. Flaws in the material, such as microscopic hairline cracks or pits, affect the localized resistivity of the material. Flaws in a material cause localized variations in the eddy currents in the material. Accordingly, a conductive material can be inspected for flaws by inducing eddy currents in the material.
Eddy current probes detect material flaws by sensing variations in eddy currents. These probes have coils with high-frequency current that project a fluctuating magnetic field into the conductive material being measured. This imposed magnetic field induces eddy currents in the material. The strength of the eddy currents depends on the local resistivity of the material, which resistivity is affected by the presence of material flaws and cracks. The eddy currents create a magnetic field that varies in intensity with the strength and, hence, the presence of material flaws.
The magnetic field created by the eddy currents extends above the material surface up to the probe. The magnetic field from the eddy current induces its own voltage in the probe coil. The eddy magnetic field opposes the coil field. These coupled magnetic fields measurably influence the net current and inductance of the probe coils, and variations in the coil currents vary in response to material flaws and are measured to detect these flaws.
Generally, the current probe is moved axially along the length of the surface to be scanned. As the probe completely traverses each scan line across the surface, the probe is circumferentially indexed to the next scan line around a reference frame. The probe is then drawn in reverse along the next scan line. This scanning and indexing sequence is repeated until the probe completely scans the entire surface. The probe must cover the entire surface to ensure that all material flaws are detected. To do this, the probe travels along straight scan lines parallel to the axis of the surface. If the probe wanders off a scan line, portions of the material surface will be missed, and flaws in the material may escape detection. Moreover, it is difficult to accurately specify the location of flaws when the probe drifts off the intended scan line.
Probe sensitivity to small flaw detection is limited by the size of the probe sense coil. The conventional small probes require long scanning times for scanning and indexing the probe over the entire surface of the part.